Given the low temperatures of Titan's surface environs chemistry is naturally somewhat sluggish. Reactions that do occur typically require a specific impetus. An example of this would be the input of energy into the upper atmosphere which drives the photolysis of methane. There are numerous other methods whereby energy can be delivered to drive chemical reactions, and these are listed here.

• Charged particles.

Cosmic rays and magnetospheric electron and protons drive dissociation reactions in the atmosphere. Where they penetrate to the surface they are able to set off reactions in exposed ices. Clathrates, for example, become darkly coloured by irradiation (Thompson et al, 1987), and Hapke (1986) has shown that sputtering of ammonia-water-ice regoliths by energetic ions yields complex hydrocarbons and carbohydrates such as monosaccharides. (See also Delitsky & Thompson, 1987.)

• Lightning.

Sizeable electrical discharges produce shock heating of the gases through which they propagate, catalyzing chemical reactions. An assessment of the amount of energy dissipated by lightning on Titan was made by Borucki et al (1984), and this was estimated to be 4x10E^-6Wm^-2, give or take an order of magnitude. Desch and Kaiser (1990) revisited the problem, analysing Voyager PRA data to determine at what level sferics* due to lightning would have become detectable. In order to reconcile the lack of observation of lightning sferics with the inferred amount of energy dissipated by lightning they found that the total flash discharges must be approximately 1000 smaller than in terrestrial lightning discharges. Navarro-Gonzalez (1997) demonstrated an alternative lightning mechanism consistent with these very low discharge rates. He found that ions created by cosmic rays and Saturn magnetospheric electrons would be separated by updrafts (transporting positively charged particles) and downdrafts (transporting negatively charged particles) having attached to cloud droplets. The weak electric fields developed in this way should produce corona discharges and weak intracloud lightning discharges. The level of energy discharge via this mechanism has interesting implications for Titan's surface organic inventory. The work of Borucki et al indicates that around 4x10E³g cm^-3 of material could be synthesized by lightning during Titan's history; this is roughly on a par with the production rate due to EUV photolysis of molecular nitrogen. Romero and Navarro-Gonzalez (1997) indicated that unsaturated hydrocarbons and nitriles are the dominant products of lightning catalysed chemistry.

• Impacts.

The passage of bolides through the atmosphere generates shock heating much as lightning does. Furthermore, the kinetic energy of the bolides is transferred to the ground upon impact, liberating a great deal of heat energy, which then initiates chemical reactions. Jones and Lewis (1987) modelled impact driven chemistry for 59 species composed of H, C, N, O, and S. One of the conclusions of Borucki et al (1984) was that shock heating of carbon bearing gases caused the condensation of graphite. Jones and Lewis found that graphite condensation is actually kinetically inhibited, and that much of the energy is channelled into creating organic compounds. Jones and Lewis calculated organic production rates for three major impactor populations that are thought to have affected Titan; the so called Population I objects derived from the proto-Saturn disk; fragments of Hyperion; and planetesimals that formed around Uranus and Neptune and were gravitationally perturbed by Saturn. Their conclusion was that organic production was only significant for the latter population (due to their high velocities and thus greater kinetic energies), this being more than sufficient to generate Titans N₂ atmosphere from the chemical destruction of ammonia (the photolysis of ammonia being problematic in their opinion).

Above: Table 1.6. A comparison of the chemical synthesis production of various mechanisms. Reproduced from tables 3 and 4 of Jones & Lewis (1987).

One problem not yet considered is the effect of impact melt on surface chemistry. Thompson and Sagan (1992), and Thompson et al (1992) tackled this directly. They determined that ~70% of the organic condensates on the surface would be exposed to aqueous solutions (probably containing ammonia) in impact craters and impact ejecta for mean periods of around a thousand years. The interaction of these liquids (Thompson et al, 1992, includes cryomagmas under this banner) with tholins is already known to yield amino acids (Khare et al, 1986). Alkynes are expected to be hydrolyzed to aldehydes and ketones, and alkenes to alcohols. HCN addition, ammonolysis, nitrile oligomerization, and aldehyde-nitrile reactions are all thought to occur, plausibly generating amino acids and nucleotide bases (Thompson et al, 1989).

Titan is widely considered to be the solar system's pre-biotic laboratory (Lunine & McKay, 1995). The production of amino acids from a mixture of reducing gases echos Stanley Miller's famous (1953) experiment. The mixture of gases in Titan's atmosphere is not too dissimilar to the Earth's atmospheric composition 4 billion years ago, and the chemical processes taking place there may have much to teach us about the way life might have begun. One of the questions we wish to have answered is whether the step from simple precursors, such as HCN to amino acids (see Oró & Lazcano-Arujo, 1981), is one that is made easily and frequently, or not at all. At the furthest limit of speculation is the question of whether there is now, or has been, life on Titan. We know full well the extraordinary range of conditionals under which living organisms survive on (and in) the Earth (the so-called extremophiles). Raulin et al (1995) investigated the extent to which pre-biotic organic chemistry could take place. Early in Titan's history, when the surface temperature was much higher, there was the possibility of pre-biotic reactions proceeding to completion on time-scales of a few years. As temperatures fell below ~200K, however, reaction half lives extended to tens of millions of years, effectively sealing surface organic chemistry in deep-freeze.

Click here to see the chemical pathways from HCN to amino acids.

Above: The importance of HCN in pre-biotic chemistry is illustrated in this figure from Hansson (1997). There is every possibility that conditions existed early in Titan's history that were capable of driving reactions as far as peptides and proteins. Conditions have probably been sporadically favourable to the creation, at least, of amino acids in meteor impacts.

Raulin et al (1995) also noted that water is not strictly essential. A pseudo-biochemistry involving ammon-analogues* of terrestrial biological molecules is possible, but the detailed chemistry has yet to be worked out.

*Ammono-analogues. These molecules replace the oxygen atoms with NH molecules. For example, an a-amino acid becomes an a-amino amidine. Similarly, there are ammono peptides and ammono ribose analogues.

Though the possibility of life at the surface is remote in the extreme in the present epoch - it's simply too cold - the ammonia-water ocean beneath the crust seems to be a friendlier environment (Fortes, 1997). Life in this medium could have developed early in Titan's history when the ocean was exposed at the surface. Energetic radiation may have promoted the synthesis of the necessary organic molecules in the same way that the process is thought to have occurred on the earth. Given the relative lack of oxygen compounds, ammono-analogues could become the basis of an alternative - alien - biochemistry. As the ocean became roofed over by an ice crust, organisms may have continued to survive (and even thrive) at great depths, and in the absence of sunlight. There are numerous groups of micro-organisms on Earth which do not rely on sunlight for energy. Of particular interest is a specific group of archaebacteria, the methanogens. Methanogens are obligate anaerobes (they rely entirely on anaerobic synthesis, unlike facultative anaerobes which are able to switch to aerobic synthesis, and they are poisoned by oxygen) that generate energy from hydrogen, by reducing carbon dioxide, methanol, methylamine, methanoate, or ethanoate, to methane. The reaction is given as 4H₂+ CO₂ " CH₄+ 2H₂O (Zinder, 1993) Methanogens are abundant in many terrestrial anaerobic environments, from the black smokers of the sea floor (many are extreme thermophiles, surviving in water at temperatures up to 118°C), to the guts of ruminants (cows produce 50 litres of methane a day through the activity of symbiotic methanogens^†). Could methanogenic organisms be a plausible 'fifth possibility' for the source of Titan's methane? This is one question we do not expect the Cassini-Huygens mission to be able to clear up.

† Thanks to Kenneth Todar at the University of Wisconsin-Madison Department of Bacteriology for that one. At this rate it would take the methanogen content of the guts of 1.3 billion cows 4.5 billion years to supply Titan's estimated methane inventory of 7x10E¹⁹kg.

Returning to impacts. Impact events are also expected to induce solid/solid phase changes in the bedrock. The terrestrial equivalent is the creation of high P-T phases of quartz (stishovite, for example) by meteorite impacts. Impacts into icy surfaces should produce high pressure phases of ice (Gaffney & Matson, 1980: Smoluchowski & McWilliam, 1984). There are fully eleven crystalline phases and one amorphous phase of water ice (see appendix II, and below), the first of which were identified as early as the turn of the century (Tammann, 1900). At low enough temperatures (below about 100K) these high pressure phases are metastable at low pressures. Thus, high pressure phases produced during meteorite impacts can survive for long periods at the surface of Titan. These other phases of ice have distinct IR absorption spectra and it may prove possible to map their distribution. No work has yet been done on the susceptibility of different crystalline forms of ice to chemical or mechanical erosion.

Phase diagram for water ice.

Hexagonal ice (ice Ih) is the ice we all know and love. Ice Ic is a cubic version of ice Ih that occurs in high level ice clouds on Earth (Whalley, 1981, 1983) and is a low temperature-low pressure phase. Ices II, III, V, VI, VII, and VIII are higher pressure phases. Ice VII, for example, only occurs at pressures greater than ~20kbar. Both ice VII and VIII are of interest because their structures are essentially those of a water clathrate. These ices form a clathrate structure in which both the 'cage' and 'guest' molecules are H₂O, accounting for their higher densities (1.50 and 1.49g cm^-3 respectively). Ice IV, discovered in 1911, is a metastable phase which occurs within the stability field of ice V. Ice IX is a low temperature metastable phase of ice III, and ice X (also known as ice VI') is a low temperature metastable phase of ice VI. Amorphous ice (or HDA-ice) is formed by the deposition of water vapour at very low temperatures. Amorphous ice spontaneously transforms to crystalline ice at temperatures above about 125K. See Whalley (1985) for a more detailed discussion of ices. See also Appendix 2.

Ground Based Observations.

Titan has been rather unfavourably located in the sky for the past decade or so for radar observations. It has been too far south to be visible to the Arecibo radio telescope and has made only short (about twice the light travel time) passes over the VLA (Muhleman et al, 1995). From October of 1997, however, Titan will be within the limited field of the Arecibo dish. The Cornell owned telescope, situated in a natural hollow in Puerto Rico, underwent some significant upgrades during 1995 and 1996. These consisted primarily of improvements to subsystems and to the interior surface of the dish, but also saw a power boost from 420kW to 1MW. The sum total of these upgrades is expected to be a factor of forty increase in sensitivity (NAIC home page, 1997).

Similarly the VLA is due for upgrades later in 1997 that should improve sensitivity by a factor of between two and fifteen, depending on the wavelength used (NRAO home page, 1997). At present the broadest configuration of the VLA (the 'A' configuration) over-resolves the disk of Titan. Smaller configuration fail to resolve the disk, and can additionally cause antenna shadowing, as well as allowing stray flux from Saturn to affect the signal. Improvements to the VLA, and possibly to the Goldstone dish (which is used in a bistatic radar arrangement with the VLA) may make it possible, in the future, to produce disk resolved radar images of Titan using the VLA-A configuration (Muhleman et al, 1995).

Important new developments in both active and adaptive optics will be used to image the surface of Titan at infra-red wavelengths, and to study the behaviour of the lower atmosphere. Both the Keck 10 metre instrument and the Multi-Mirror Telescope will be heavily involved in these investigations. Many new observations using these powerful optics will be made in the 2µm window where the haze opacity is lower.

In space we have the relatively new Infra-red Space Observatory, launched in December 1995, and a new infra-red camera was recently installed aboard the Hubble Space Telescope. This new camera (NICMOS) will be utilised to observe and resolve surface details in the 2µm window (something Hubble was previously unable to do), to help constrain the IR spectrum of surface materials, and to search for cloud features (Lemmon, 1997).

Perhaps the most exciting discoveries will come with the arrival of the Cassini space probe at Saturn in 2004. The mission is a joint NASA-ESA project to place an orbiter about Saturn and a lander on the surface of Titan. NASA is primarily responsible for providing the Cassini orbiter (though ESA countries are supplying a number of key instruments), and the ESA is responsible for the construction of the descent probe.

The Cassini mission was conceived in the early 80's, in the wake of the Voyager space-crafts' joint successes. A very ambitious probe was initially designed. Budget cuts have since required that Cassini's grand design be scaled down. In early 1992 a number of scientific objectives for the cruise to Saturn were dropped (observations during the Jupiter flyby, for example) and there were several instrument design alterations. Instead of swivelling instrument platforms, as were built onto the Voyagers, many of the experiments are now fixed to the probe chassis (Bertotti, 1992: Pernice, 1992: Lebreton & Matson, 1992).

Cassini was launched on October 15th 1997, and will spend seven years en route to Saturn. During these seven years it will make flybys of Venus (twice), Earth, and Jupiter, arriving in Saturn orbit on July 1st 2004. Its first orbit will permit a close flyby of Titan on November 27th 2004, a flyby timed to coincide with the entry of the Huygens probe into the atmosphere of Titan. The probe will be released 21 days prior to entry and will coast to its atmospheric entry point. Cassini will then go on to conduct an incredibly complex four year tour of the Saturnian system. The final tour has yet to be decided, and in fact will not be finalised, and the software required to implement it uploaded, until very late in the cruise to Saturn. The tours currently being looked at call for between 30 and 60 orbits over the four years to 2008. Some are likely to be very short orbits (~10 days), others will be considerably longer (~100 days). Furthermore the inclination of the orbits will be varied (a procedure known as 'cranking') to sample high magnetospheric latitudes and to look down on the ring plane. Orbits will be switched by making use of Titan's gravitational field. This yields up to 40 close flybys of Titan. With the possibility of extending the four year mission, perhaps for a further four years to 2012, the potential for study of Titan is clearly enormous.

The objectives of the Cassini mission at Titan are to characterize;

• The composition of the atmosphere.
• Energy sources for atmospheric, and surface, chemistry.
• Aerosol properties, hazes and clouds.
• Global atmospheric circulation, wind speeds and temperatures.
• The internal structure of the satellite.
• The state (solid/liquid), chemical composition, and morphology of the surface.

These will be achieved using the instruments listed in the table below.

Table 1.7

The Huygens probe, having been floating free for 21 days will enter the atmosphere of Titan at an altitude of ~1270km. The high drag heat shield will protect the descent probe during the period of peak decelaration. During this entry period only one instrument, the Huygens Atmospheric Structure Instrument, will be operating. At an altitude of ~190km a pilot chute is to be released that is designed to brake the landers' velocity from Mach 1.5 to about 100m/s. One second later and ten kilometres lower, the main parachute will be deployed, followed thirty seconds later by the separation of the heat shield. Here, at an altitude of ~170km, the instruments are deployed and measurements of the environmental conditions are recorded and relayed back to the Cassini orbiter for transmission to Earth. The main parachute is discarded after roughly 9 minutes and a much smaller stabilizing chute is deployed. The next phase covers the 150 minutes (this figure is an absolute maximum) that it will take the descent probe to reach the surface; impact being at a velocity of around 5ms^-1. Assuming it takes the full 150 minutes to arrive and the instruments survive, there will be sufficient electrical power for three minutes of surface science. Of course a shorter flight means more (perhaps as much as thirty minutes more) activity on the ground. The probes targeted landing site is illustrated on the map below, a spot located on the edge of the leading hemispheres high albedo patch. Uncertainties in winds speeds, though, means the probe may be blown as much as 1000km off course.

Map showing Huygens landing site.

The composition of the atmosphere will be determined using the infra-red, visible, and ultraviolet spectrometers (CIRS, UVIS, & VIMS), and this information will be supplemented with occultation data from the radio science subsystem. This remote sensing data is to be augmented by in situ measurements from Huygens' gas chromatograph mass spectrometer, which is capable of detecting species down to the 10E^-12 mixing ratio level.

The energy sources for atmospheric chemistry will be assessed by the charged particle experiments aboard Cassini (CAPS, INMS, & RPWS). These will measure the flux of magnetospheric electron and protons, and cosmic rays, in the vicinity of Titan. The HASI experiment on the lander will also detail the electrical conductivity and ambient electric field strengths during its descent. Properties of the aerosols, hazes and clouds, can be studied from space by the array of spectrometers on Cassini, by visible light photometry, and observation of the hazes scattering properties. In addition the Aerosol Collector & Pyrolyzer (ACP), on Huygens, will collect atmospheric samples, pyrolyze them in a small oven, and deliver the resultant gases to the GCMS instrument for analysis (Frère et al, 1990: Ehrenfreund et al, 1995). Visual searches for clouds will also be carried out, not only by the imaging systems on the descent probe (DISR) but by the VIMS and ISS instruments in Cassini, in the appropriate wave bands. Lastly, the HASI experiment ought to be sufficiently sensitive to slight changes in the temperature/pressure profile to detect the condensation of clouds (Fulchignoni, 1992).

The atmospheric structure will be most closely studied by the HASI instrument suite (Fulchignoni, 1992: Ferri et al, 1997). This suite is composed of four major subsystems:

This monitors decelaration during descent (Fabris et al, 1992), and upon impact (Lorenz, 1994b).

The Pressure Profile Instrument (PPI), developed by the Finnish Meteorological Institute, will yield the most accurate pressure profile yet, and provide ground truth for radio occultation experiments.

The HASI temperature sensor is designed to operate in the lowermost 150km of the descent, when speeds are below Mach 0.6. Resolution of 0.02K is anticipated (Cornaro et al, 1992). The determination of the thermal structure of the atmosphere will be vital to understanding the global climate, and the global circulation. In conjunction with pressure data, this instrument will define an adiabat, which is pivotal in answering the question of whether the atmosphere is in radiative equilibrium, and if there is a convective zone. The radiative balance can be found by combining information from this sensor with the results from the spectral radiometer.

This will measure the electrical conductivity of the atmosphere and the ambient DC electrical field. Antennae will also search for emissions between 1 and 10Hz which might be characteristic of lightning bursts. A microphone will listen for associated acoustic phenomenon, such as thunder or noise due to turbulence (Grard et al, 1995). As well as measuring the electrical properties of the atmosphere the PWA, should it survive landing, will measure the electrical properties of the surface.

Atmospheric structure will be studied by the RSS package on Cassini as well as the HASI package on Huygens. Repeated radio occultations will give us the opportunity to derive P-T profiles for varying latitudes, and aid in the development of a Global Circulation Model.

Wind speeds will be measured directly by the Doppler Wind Experiment (DWE). Probe doppler tracking by the orbiter should yield a zonal wind profile. Furthermore, should the search for clouds prove successful, then winds at altitude can be monitored by tracking the motions of these clouds.

Plans for studying the internal structure of Titan involve measuring the gravitational potentials acting on the Cassini probe as it flies by the satellite. The methods involved in this are discussed in Comoretto et al (1992) and Rappaport et al (1997), and have already been touched on here.

The surface will be remotely sensed by a range of instruments. VIMS will give near global coverage of the surface in the 0.94, 1.07, 1.28, 1.58, and 2µm atmospheric windows with a resolution of approximately 500 metres. Where the imaging science system can see the surface (a matter still open to question), resolution may increase to 30 metres. Stereographic mapping using these two system can yield valuable information on topography, to supplement the radar altimeter. However atmospheric scattering is expected to diffuse the solar illumination to such an extent that photoclinometry by shadow measurement is likely to be ineffective (Baines et al, 1992: Lorenz, 1995). The descent imaging system carried by the Huygens probe is capable of imaging the surface at far higher resolutions. The last pictures are expected to be taken about 150 metres from impact. The anticipated field of view at this height is approximately 10,000 square metres, and the resolution approaching 20cm. To combat the poor illumination (surface brightnesses being likely to be no greater than a moonlit night on Earth) Huygens is equipped with a powerful lamp. Pictures will also be taken of the horizon, not only to give a feel for the true Titanian landscape, but more importantly to measure the optical depths of surface hazes.

Cassini is also going to be flying a sophisticated radar mapping system. The 50W system consists of a 13cm wavelength transmitter and receiver package that can work in several modes (Elachi et al, 1991: Picardi et al, 1992):

1) Altimetry, to produce topographic profiles.

2) Real aperture side-looking radar to achieve wide image swaths.

3) Synthetic aperture radar to generate high resolution images.

4) Radiometry, to determine the emissivity and dielectric constants of surface materials.

(See Ford et al, 1993, for a clear explanation of the uses of space-craft borne radar systems.)

Radar images of the surface, of comparable quality to Magellan images of Venus (though of a slightly lower resolution), will be returned for between 10 and 30% of Titan. Needless to say, the exact figure depends on the choice of orbital tour and hence the number of flybys of Titan. An extended mission opens up the possibility of more extensive radar mapping, perhaps with Cassini placed in a permanent orbit around Titan.

Thompson and Squyres (1990) conducted a very detailed investigation of the dielectric properties of ices and hydrocarbons (liquid and solid), with a view to evaluating the efficiency of the Cassini radar for characterizing surface composition. Their calculations show that, for wavelengths longer than about 10cm, any surface liquids become sufficiently transparent for an echo signal from the seafloor interface to become detectable. Furthermore, the loss tangent* in ice is very low, so echos received by the Cassini radar could quite well come from buried sub-surface interfaces (cf., detection of buried river beds in terrestrial deserts by orbiting SAR). Thompson and Squyres note also that, if the surface layers - hydrocarbons or regolith - are shallower than about 200 metres, then the Cassini radar may well be unable to resolve them.

Even so, radar reflectivites of water ice and liquid hydrocarbons differ by a factor of approximately 10, so the Cassini radar is very likely to detect bodies of surface liquids, be they lakes or oceans, quite easily. The radar may even by called upon to act as a wind scatterometer, measuring wind speeds from the roughness of the sea surface.

Finally, the Huygens probe is hoped to survive its touchdown and continue to function for at least three minutes. While there it will work to add to the mountain of information on the surface being accumulated by the orbiter (Lorenz, 1992). The ACP/GCMS will continue to analyse the composition of the surroundings. If the probe lands at sea, instruments in the surface science package will measure density, refractive index, and thermal and electrical conductivity. The accelerometer will also be able to tell if the probe is bobbing up and down (if in liquid), and an acoustic sounder will measure the depth of the liquid (Zarnecki, 1992). An impact penetrometer should allow constraints to be placed on the particle size and cohesion of the surface layers (Lorenz et al, 1994), and the possible generation of a dust cloud could give further useful clues as to the cohesion of the surface and availability of loose dry sediment (Lorenz, 1993c). If the surface is solid, the accelerometer may be used to infer the density, porosity, and cohesion of the target (Lorenz, 1994b). Using each of these different methods to study similar surface attributes should permit close constraints on the characterization of surface materials.

It should be clear by now. There's a lot we know about Titan. There's a great deal more we don't know. There are a great many questions that the Cassini mission will answer. There will, no doubt, be a whole lot more questions that it causes us to ask.

Cassini cannot (and nor should we realistically expect it to) furnish us with all the answers we demand. It is not able to dig cores, return samples, study seismicity, or even accurately map all of the satellite. The Cassini mission is, of necessity, a planning compromise between the needs of the scientists wishing to study, for example, Saturn's magnetosphere, or those who wish to study the planets' other satellites (yes! Saturn does have 17 other moons apart from Titan), and those who see Titan as Cassini's true goal. This is as it should be.

Nevertheless it seems certain that we can anticipate a time in the future when there will be a Titan orbiter, a Titan lander, a Titan rover, and just maybe, a Titan colony. A number of designs exist for a new generation of nuclear powered rocket engines (so-called NERVAs - Nuclear Engine for Rocket Vehicle Applications) that can make use of a wide range of fuels such as water, carbon dioxide, and methane. Sir Arthur C. Clarke was the first to propose such rockets for the mass exploration of the solar system in the 1960's. NASA studied the idea briefly and dropped it in 1972 as it simultaneously shrank from continued lunar exploration. However the idea has been given new life (Zubrin, 1990) as a means of getting to Mars, and exploring the outer solar system on reasonable time scales. The postulated existence on Titan of such an enormous reservoir of methane makes it a logical stopping point for outbound vessels based on NERVA technology.